The present invention relates to a phase modulation device for an ophthalmic instrument using a light beam interacting with an eye. It also relates to ophthalmic instruments implementing such a device, as well as to a calibration process for these ophthalmic instruments.
The field of the invention is more particularly that of visual correction simulators and ophthalmic imaging devices, in particular for high-resolution retinal imaging.
Ophthalmic instruments, such as for example instruments for retinal imaging or for optical laser treatment on the retina, operate with a main beam intended to pass through the various optical elements (cornea, crystalline lens, etc.) of a patient's eye, either by a beam incident on the eye (case of optical treatment of the retina), or as a beam emerging from the eye (case of retinal imaging).
In all cases, aberrations of the different optical elements of the eye cause aberrations of the wave front of the main beam, which degrades the quality of the optical instrument. Thus, in the case of retinal imaging, the image loses resolution and, in the case of the optical treatment of the eye, the quality of focusing the laser on the retina is degraded.
It is known to combine these ophthalmic instruments with a system for correcting aberrations of the eye making it possible to correct the wave front of the main beam, i.e. to give the optical beam phase the closest possible shape to a predetermined shape making it possible to obtain optimal performances from the instrument.
Such a system can also be used in an ophthalmic instrument of the vision simulation type, the purpose of which is to show a patient the effects of different corrections (corrective lenses, contact lenses, refractive surgery) by making him “see” an image, the analysis beam incident on the eye of the patient being then corrected for the ocular aberrations and/or showing the optical effects induced by the phenomenon which it is desired to simulate.
Such a system for correcting aberrations, described for example in patent application FR 2 866 551, comprises a Shack Hartmann type analyzer for measuring aberrations of the eye and an optical device for modulating the phase of the wave front of the main beam, of deformable mirror or spatial light modulator type, and controlled for correcting the wave front as a function of measured aberrations of the eye. In these systems, control of the optical modulation device is calculated as a function of a desired phase modulation on the main beam.
In the case of a visual simulation appliance operating in a closed loop, the problem generally faced is the need to direct light into the patient's eye, resulting in dazzling and thus disturbance to the simulation process, as well as creating a sensation of ocular discomfort.
In order to overcome this problem of dazzling in a closed loop, it is possible to use an incident beam in a spectral field invisible to the eye, typically in the infrared, but this would then involve dependence on the chromatism effects linked to a significant wavelength discrepancy between the aberration measurements (carried out in the infrared spectra) and the visual stimulation (carried out in the visible spectrum). It could also be envisaged to operate the modulation means in an open loop (without feedback). In this case, no light is directed into the patient's eye, and consequently, no wave front measurement is possible since no light returns from the eye. Such an approach has the drawback of imperfection of the modulation means (linearity problem, hysteresis, temperature drift). The errors generated on the modulation by these defects are not compensated for by a feedback loop (closed loop) and thus degrade the results of the correction or simulation.
Moreover, a further problem resides in the need to provide a learning or calibration stage of the adaptive optics process, the periodicity of which typically depends on the precision and stability requirements of a given application. In practice, this learning stage is carried out by using an artificial eye, subject to the following restrictions: said artificial eye must have a very high optical quality and requires careful alignment in front of the instrument.
The problem also arises of overcoming aberrations of the optical system itself. In fact, in many applications in which it is desired to perform measurements of eye aberrations in order to carry out correction or simulation, it is important to overcome the aberrations of the optical system used, as it must be capable of distinguishing on the one hand, the aberrations of the optical system of the ophthalmic system and on the other hand, the aberrations arising from the eye. A single measurement of the system aberrations carried out during its set-up is generally insufficient to ensure that these aberrations are known over a long period (typically several months) due to their temperature-dependence. In fact, the phase shifter element is generally an optical element that is sensitive to the temperature of its environment. Thus awareness of, or overcoming, aberrations of the optical system of the appliance requires measurement under its operating conditions. In practice, this measurement must be carried out with strict regularity, and in the state of the art requires the use of an artificial eye.
Moreover, it may be found necessary to add further corrections when it is desired to simulate phase objectives, such as an intraocular lens or implant. Here again, there is a need to overcome aberrations of the ophthalmic simulation device. In fact, in order to be capable of generating, in the pupil of the eye, a phase modulation representing the desired modulation free of aberrations of the ophthalmic device's optical system, it is necessary to be able to overcome the aberrations of the latter.
The purpose of the invention is to overcome these problems by proposing an ophthalmic device in which a complete closed-loop adaptive optics process, also comprising a learning stage, can be easily implemented without the need for the frequent use of an artificial eye, without discomfort for patients and under optimal conditions for overcoming the aberrations, despite ambient temperature variations, non-linearities and hysteresis phenomena on the phase modulators.